1. Field of the Invention
The invention relates to a light pick-up device of an opto-electronic scanning element for either transparent or opaque masters for either flat bed or drum scanning devices.
2. Prior Art
Flat bed or drum scanning devices are employed, for example, in facsimile transfer technoloy. A light beam scans the master or original, which is to be reproduced, point-wise and line-wise. The scanning light coming from the master or original strikes an opto-electronic scanning element which converts the light into an image signal. The image signal is forwarded over a transmission channel to a facsimile recording device which will record a reproduction of the master.
The masters to be reproduced may be either type masters, rastered or unrastered image masters or masters which are assembled of both type and images, and are so-called paste-up montages.
In a flat bed scanning device in which a light beam is deflected along a line of a given length, the opto-electronic scanning element is designed in such a manner that its light pick-up surface extends at least over the given line length so that approximately the same pick-up condition for the modulated light coming from the master prevails in every position of the deflected light beam.
The opto-electronic scanning element can consist of a row of photodiodes in the form of a photodiode strip which is disposed in the light pick-up surface. However, the scanning element can also be an optical shape converter, which is constructed of a multitude of optical fibers and a discrete opto-electronic transducer, for example, a photomultiplier. In this arrangement the surface of the optical shape converter with the greater expanse forms the light pick-up surface and the discrete opto-electronic transducer is coupled to the surface with the smaller expanse.
As a result of scratches in the master and as a result of so-called shadow edges in the case of a master which is a paste-up montage, the scanning light is diffusely reflected from the master with an asymmetrical intensity distribution and scanning errors will occur.
A flat bed scanning device with a scanning element for an opaque original or master is already known and is described in U.S. Pat. No. 4,080,634, whose disclosure is incorporated by reference thereto. The scanning element of this patent has a light pick-up device which, in order to reduce the aforementioned scanning errors, collects the modulated light which is reflected from the opaque master and supplies it to a photodiode strip.
The known light pick-up device, which extends over the line length, has light openings, which are oriented in the line direction on the side facing away from the master and on the side facing the master the device has a reflector for deflecting or reflecting the modulated light onto the light opening. The inside space of the light pick-up device is mirrored and has an elliptical cross-section on a plane which is perpendicular to the line direction. The light pick-up device is disposed in such a manner that the one focus line of the ellipse coincides with the point of incidence of the scanning light beam on the master. The modulated light reflected by the master is reflected by the mirrored inside surface into the other focus line of the ellipse on which the light pick-up surface of the photodiode strip is situated.
This known light pick-up device has the disadvantage that such a photodiode strip is relatively insensitive and slow, and therefore only a low scanning rate can be achieved. A higher or faster scanning rate could be achieved, for example, by employing faster photomultipliers. The disposition of a multitude of photomultipliers instead of the photodiode strip would be difficult in the known light pick-up device and would hardly be possible for reasons of space requirements. In the known light pick-up device, even the attempted use of an optical shape converter instead of the photodiode strip would have considerable disadvantages and problems. Since the angle at which an optical fiber of the optical shape converter can pick up light is significantly smaller than the angle at which a photodiode strip can pick up light, only a small part of the mirrored inside surface would be actively used in the known light pick-up device. Thus the modification of using the converter for the photodiode strip would reduce the sensitivity and uniformity of the scanning.
A further disadvantage is that the known light pick-up device is only suitable for scanning opaque masters in a flat bed scanning device and is not suitable for transparency masters and for use in a drum scanning device.